Breccia Unit Research Articles

The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations

Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions.

Imaging the Chicxulub Central Crater Zone from Large-Scale Seismic Acoustic Wave Propagation and Gravity Modeling

Large impact structures are characterized by peak ring and central uplifts with lateral/vertical mass transport during late formation stages. Here we investigate the Chicxulub crater, which has been surveyed by an array of marine, aerial and land-borne geophysical methods. Seismic reflection surveys in its central sector have shown lack of resolution, making it difficult to image the central uplift. We develop an integrated seismic and gravity model for the structural elements, imaging the central uplift and melt and breccia units. The 3D velocity model built from interpolation of seismic data is validated using perfectly matched layer seismic acoustic wave propagation modeling, optimized at grazing incidence using the shift in the frequency domain. Modeling shows that lack of illumination relates to seismic energy that remains trapped in an upper low-velocity zone corresponding to the carbonate sediments, upper melt/breccias and surrounding faulted blocks. After conversion of seismic velocities into a volume of density values, we apply parallel forward gravity modeling to constrain the size and shape of the central uplift, which has a ~ 40 km diameter concave upwards top lying at ~ 3.5–4.5 km depth. The preferred model provides a high-resolution image of crater units and structure. The gravity response of modeled units shows asymmetries in structure and the distribution of breccias, melt and target carbonates. Finally, we apply an adjoint reverse time migration approach for seismic imaging using the density and velocity models built for the acoustic wave propagation and gravity modeling, which allows improved modeling of the crater structure.

Limestone Facies Change of Jonggrangan To Sentolo Formation in The Western Part of Yogyakarta-Central Java Basin

The research location is in the western of Yogyakarta-Central Java Basin, where limestone of Jonggrangan and Sentolo Formation are found with the same age but have different depositional facies characteristics. These differences will be interesting to study the historical process of its formation. The research was conducted through a series of methods. Starting from geological mapping to determine the order of rock stratigraphy, the depiction of outcrop profiles, and limestone sampling for petrographic analysis. Eventually, the depositional environment of the two rock formations was discovered. Field observation results and laboratory analysis were further integrated to determine the rock facies that make up the two formations. The rock units in the study area from old to young are the pyroxene andesite breccia unit of Upper Oligocene age and limestone. The bioclastic limestone and layered limestone units are deposited by interfingering, unconformity over the pyroxene andesite breccia unit. The Middle Miocene bioclastic limestone unit is equivalent to Jonggrangan Formation in the inner shelf environment (Restricted Lagoon and Open Lagoon facies environment). Whereas layered limestone of Middle Miocene age is equivalent to the Sentolo Formation in the lower slope environment (Restricted Lagoon, Open Lagoon, and Open Sea Shelf) facies.

Early Cretaceous glacial environment and paleosurface evolution within the Mount Painter Inlier, northern Flinders Ranges, South Australia

Sediments comprising the Radium Ridge Breccia, a stratigraphic unit recognised within the Mesoproterozoic Mount Painter Inlier, are now re-interpreted as Lower Cretaceous (previously interpreted to be Paleozoic) periglacial deposits with associated glacial and pro-glacial facies that extend into adjacent Eromanga Basin sediments. The interpretation is based on distinctive periglacial, glacial and pro-glacial facies, clast lithology, geochronology and consistent stratigraphic position above the Mesoproterozoic basement. Facies recognised are kame deposits, tillites, talus, mass-flow and solifluction lobes. The depositional model presented herein is that the Radium Ridge Breccia unit formed in a glacial/periglacial environment in moderately elevated highlands, in a proximal, mainly terrestrial setting adjacent to the Eromanga Basin, during the Early Cretaceous. LA-ICPMS U–Pb geochronology of the distinctive Sprigg Tillite Member, a lodgement tillite within the Radium Ridge Breccia unit, yielded the youngest zircon ages ca 240–220 Ma and monazite ages ca 365 Ma that are consistent with the interpretation of this subunit, and hence the Radium Ridge Breccia formed during the Early Cretaceous. The Sprigg Tillite Member is correlated with the early Aptian Sheehan Cretaceous glacial event.

Detailed mapping and lithogeochemistry of Neoarchean volcanic rocks: Beaulieu volcanic belt, Slave Craton, NWT

Neoarchean volcanic rocks of the Beaulieu River volcanic belt structurally overlie basement rocks of the Sleepy Dragon Complex (ca. 2.85 Ga), approximately 100 km east northeast of Yellowknife. The volcanic belt is comprised of complex lithofacies, including basalt, andesite, rhyolite, and associated volcaniclastic rocks, and hosts the Sunrise volcanogenic massive sulfide deposit. The absolute age of the volcanic strata is not known, nor is the stratigraphy well-defined; therefore, the Beaulieu River volcanic belt cannot be easily correlated to other greenstone belts within the Slave craton. The main objective of this study is to document the litho- and chemo-stratigraphy of the volcanic rocks, and particularly the rhyolite dome, located at the south end of Sunset Lake to reconstruct their volcanic and petrogenetic evolution, and determine their relationship to the volcanic strata that hosts the Sunrise VMS deposit, located ~6km to the north of the study area. Detailed mapping (1:2000) was completed over two field seasons (2018 and 2019) and shows that the volcanic rocks in the south Sunset Lake area comprise a complex stratigraphy consisting of basaltic, andesitic and rhyolitic lithofacies. This includes massive to pillow basalt and andesite, with lesser amounts of massive to in-situ brecciated, weakly quartz-plagioclase porphyritic rhyolite, heterolithic tuff to lapilli- tuff and felsic tuff to tuff breccia. The felsic clasts within the felsic volcaniclastic rocks are similar in composition to the coherent rhyolite. Units have a trace element geochemical signatures that vary from tholeiitic to calc-alkaline, arc-like rocks. Volumetrically, the volcanic strata in the south Sunset Lake area has a significant amount of volcaniclastic rocks, ranging from tuff to tuff breccia units. The volcaniclastic rocks are interpreted to have been deposited by a series of debris flows and eruption-fed density currents. The stratigraphy of the volcanic rocks in south Sunset Lake is very similar to that of the stratigraphy that hosts the Sunrise VMS deposit. Evidence of a vent proximal environment (e.g. rhyolite dome, peperite, syn-volcanic intrusions) and porous, volcanic debris accumulating on the seafloor highlight conditions favourable for volcanogenic massive sulfide-type mineralization in the south Sunset Lake area.

Brittle overbreak prediction in deep excavations for hydrothermally altered and heterogeneous rockmasses

In massive rockmasses under high in situ and deviatoric stresses, brittle spalling is the dominant damage process and failure mode and can result in significant depths of overbreak outside the excavation design dimensions during and after construction. Significant work has been done by researchers to develop empirical and mechanistic prediction tools for brittle overbreak in homogeneous rockmasses. Brittle overbreak in heterogeneous rockmasses that contain hydrothermal veins and breccia can behave quite differently than homogeneous rockmasses, with the result that the existing tools for homogeneous rocks are inadequate. In this study, overbreak profiles and excavation observations from part of the undercut level in the New Mine Level project at the El Teniente copper–porphyry mine in Chile and intact laboratory test data from the four local rockmass units are used to develop new functions based on brittle rock mechanics to improve the prediction of the depth of brittle overbreak for similar excavations in heterogeneous rockmasses. The dacite porphyry unit, which has few stockwork veins, most closely resembles the expected homogeneous brittle overbreak development. The stockwork mafic complex unit exhibits overbreak that exceeds the homogeneous prediction because the planar, pervasive quartz veins act as crack attractors and develop maximum crack propagation after initiation and immediate strength loss. The anhydrite breccia unit and a brecciated contact zone contain large clasts of contrasting mineralogy that act as crack arresters, resulting in minimal overbreak. The proposed functions to predict depths of brittle overbreak around tunnels in these heterogeneous rocks can ultimately be used to aid in excavation and ground support design.

ANALISIS STRATIGRAFI DAN RUMUSAN SEJARAH GEOLOGI DAERAH LIMBATO DAN SEKITARNYA, KABUPATEN GORONTALO UTARA, PROVINSI GORONTALO

Regionally, the research area is composed of volcanic and plutonic rocks. Stratigraphic data of the study area are still regional scale. This study aims to analyze detailed stratigraphic data and formulate the geological history of the Limbato area and its surroundings. The area located at coordinates 0º56’6” - 1º0’10.8” N dan 122º5’56.4” - 122º9’10.8” E, the area of the study is around 45 km². The method used in the research is scientific exploration by conducting a surface geological mapping survey. The geological surface in the form of field data collection and laboratory analysis. Field data in the form of lithology data and laboratory analysis in the form of petrology and petrographic analysis. The stratigraphy of the research area is divided into 4 (four) unofficial units, which are sorted from old to young, namely granodiorite units of Middle Miocene age, volcanic breccia units of Late Miocene to Early Pliocene, andesite units of Middle Pliocene and alluvial units of Holocene age.Stratigraphic analysis of the Limbato area is related to geological history. In the Middle Miocene period subduction formed plutonic rocks that produced granodiorite units, then during the Late Miocene volcanic eruptions resulted in volcanic breccia units. The activity was stopped at the time of the Early Pliocene. After the volcanic activity was stopped, then new magma activity occurred at the time of the Middle Pliocene resulting in an andesite unit. After all rock units have been deposited, an erosion process occurs in older units which results in an alluvial unit.

Late Cretaceous to early Eocene geological history of the eastern Ionian Basin, southwestern Greece: A sedimentological approach

Sedimentological studies of the Upper Cretaceous–lower Eocene limestones in the western part of the Ionian basin (Araxos area) indicate that these deposits are composed of calciturbidites interbedded with breccia-microbreccia. Breccia - microbreccia deposits are structureless, display channelized geometry with calciturbiditic blocks internally to the channels. Most of the clasts were sourced from the underlying Lower Cretaceous “Vigla limestones”. Calciturbidites include Ta to Te Bouma sub-divisions, and are organized in cycles that form channelized deposits with a high degree of amalgamation. Statistical analysis confirms the presence of order in the sub-division sequence. The thickness of event beds in the studied section shows a lognormal statistical distribution, typical of turbidite successions. Limestone microfacies suggest deep-water deposits and reworked shelf deposits.The intense extensional tectonic activity in the Ionian basin during the Early Cretaceous, with synthetic and antithetic faults, produced active platform margins and asymmetrical grabens. In this regime, large amounts of coarse-grained material became available and accumulated in the basin. High slope gradients led to slumping. During the Late Cretaceous, the uplifted margins of the grabens caused erosion of the pre-existing deposits of the Lower Cretaceous “Vigla Formation”. This event led to the accumulation of channelized microbreccia and breccia units and transport of platform deposits by turbidity currents. The Early Cretaceous to early Eocene depositional history in the Ionian Basin indicates that the regional tectonic activity, rather than the eustatic sea-level changes, was the major factor that influenced the basin evolution, suggesting a syn-rift stage being active from the Jurassic to the early Eocene.

Geologi Daerah Geothermal Pangi Kabupaten Bone Bolango Provinsi Gorontalo

Geothermal is associated with volcanic that still has heat energy below the surface. Indonesia is a country with geothermal energy reserves of around 40%. There are 256 potential geothermal areas and only 7 locations have been utilized. Gorontalo has several geothermal manifestations, such as Lombongo, Pangi, Libungo, and Hungayono. The purpose of this study was to study the geology of the geothermal manifestations of Pangi, East Suwawa District, Bone Bolango Regency, Gorontalo Province. The research method is divided into field observations, laboratory analysis, and studio analysis. The results of this research show that the geomorphology in the study area is divided into 3 units, namely the Fluvial Plain, the Volcanic Hills, and the Structural Use Landscape. Alluvial Deposition Unit, Volcanic Breccia Unit, Tuff Lapili Unit, Andesite Unit, Granodiorite Unit, and Porphyry Diorite Unit.

Igneous architecture and implications for diverse Cu-PGE mineralization styles in a conduit system: an example from the Area 41 Cu-PGE occurrence, Coldwell Complex, Canada

The Area 41 copper-platinum-group element (PGE) occurrence was discovered in 2006 at the northwestern margin of the Coldwell Complex. The discovery was significant because of the sizable mineralized intersections, including 1.4 ppm Pd + Pt and 0.32% Cu over 43 m. Since then, exploration along the northern margin has resulted in the discovery of several other Cu-PGE zones with similar grades and host rocks. Field relationships, drill-core logging, petrography, and lithogeochemistry indicate that the rocks formed from several extrusive and intrusive events related to at least three magmatic series that are from oldest to youngest: Metabasalt, the Layered Series, and the Marathon Series. Three types of Cu-PGE mineralization are hosted within separate intrusions of the Marathon Series. Type 1 is hosted by coarse-grained subophitic gabbro and exhibits high Cu/Pd (> 100,000) and low PGE grades, formed at R-factors of 105) and low S/Se ratios (800–2000) that suggest S loss. Three-dimensional modeling of the occurrence shows a north-south, keel-shaped trough at the western end of mineralization and a systematic spatial variation in the proportions of various Marathon Series units, including pegmatitic subophitic gabbro, breccia units, apatitic clinopyroxenite, and oxide melatroctolite, that correlate with decreasing thickness of mineralization, eastward and away from the trough. These observations suggest that multiple intrusions were emplaced along a feeder channel, some of which flowed laterally outwards to form sills. Taken together, these characteristics are consistent with a conduit model for deposit formation and are thus important vectors for exploration.

Stability assessment of the Crater Lake/Te Wai-ā-moe overflow channel at Mt. Ruapehu (New Zealand), and implications for volcanic lake break-out triggers

The presence of lakes on active volcanoes gives rise to several unique volcano-hydrologic hazards, including those directly related to volcanic activity (e.g. phreatic explosions) and non-volcanic phenomena (e.g. lake outbursts). Mt. Ruapehu volcano hosts the 107 m3 Crater Lake/Te Wai-ā-moe in the active summit crater, making it prone to lahars due to the abundance of water stored at high elevation. Apart from eruptive periods when water is commonly ejected from the lake, large lahar events can occur when portions of the crater rim fail during quiescent periods, leading to partial lake drainage; the latter occurred recently in 1953 and 2007. Here, we investigate the potential of future lahars due to rim failure at the current overflow channel considering several triggering scenarios during non-eruptive periods. The geomechanical properties of a matrix-dominated breccia unit composing part of the overflow channel were analyzed using physical and mechanical laboratory methods. These properties, combined with previous geomechanical studies of lava units in the overflow channel, were used in finite element and limit equilibrium method-based groundwater and stability models. Groundwater models indicate that a low-permeability barrier inside the crater rim is preventing water from seeping through the unconsolidated and potentially permeable breccia unit of the overflow channel. The removal of this barrier, or weathering of a resistive lava cap, could cause water to seep through the overflow channel, elevating the potential of collapse. Slope stability models indicate that under certain conditions, the overflow channel is below a conservative threshold of stability and can be destabilized with continued mechanical or chemical weathering that reduces strength properties. This is also true during high magnitude tectonic or volcanic earthquakes, where large ground accelerations could result in several centimeters of displacement at the overflow channel, corresponding to high hazard categories of stability. In the worst-case scenario modeled here, full collapse of the overflow channel could result in the release of c. 2.4 million m3 of lake water. This could drive a lahar substantially larger than well-documented historic events, including the more common lahars produced during eruptions. The identification of key factors affecting stability can be used to inform future monitoring and the risk of these high impact events.

The origin of a coarse lithic breccia in the 34 ka caldera-forming Sounkyo eruption, Taisetsu volcano group, central Hokkaido, Japan

The 34 ka Sounkyo eruption produced 7.6 km3 of tephra (~5 km3 DRE) as fallout, ignimbrite, and lithic breccia units, forming a small, 2-km-diameter summit caldera in the Taisetsu volcano group, Japan. The Sounkyo eruption products are made up of five eruptive units (SK-A to -E) in proximal regions, corresponding to the distal deposits, a 1- to 2-m-thick pumice fallout and the Px-type ignimbrite up to 220 m thick. The eruption began with a fallout phase, producing unstable low eruption columns during the earlier phase to form a <7-m-thick succession of well-stratified fallouts (SK-A1 and the lower part of the distal fallout). The eruption column reached up to 25 km high (subplinian to plinian) and became more stable at the late of the phase, producing a < 60-m-thick, pumice-dominated fallout (SK-A2 and the upper part of the distal fallout). The second phase, the climax of the Sounkyo eruption, produced a widespread, valley-filling ignimbrite in both proximal and distal regions (SK-B and the Px-type ignimbrite). At the end of the climactic phase, the waning of the eruption led to extensive failure of the walls of the shallow conduit, generating a dense, lithic-rich, low-mobile pyroclastic density current (PDC) to form a >27-m-thick, unstratified and ungraded, coarse lithic breccia (SK-C). The failure in turn choked the conduit, and then the eruption stopped. After a short eruptive hiatus, the eruption resumed with a short-lived fall phase, establishing an eruption column up to 16 km high and producing a <6-m-thick scoria fallout (SK-D). Finally, the eruption ended with the generation of PDCs by eruption column collapse to form a 5- to 15-m-thick ignimbrite in the proximal area (SK-E). Volume relationships between the caldera, ejected magma, and ejected lithic fragments suggest that the caldera was not essentially formed by caldera collapse but, instead, by vent widening as a consequence of explosive erosion and failure of the shallow conduit. The dominance of shallow-origin volcanic rocks in the lithic fraction throughout the Sounkyo eruption products implies the development of a flaring funnel-shaped vent. Hence, the occurrence of lithic breccias within small caldera-forming eruption products does not necessarily reflect either the existence or the timing of caldera collapse, as commonly assumed in literature. Lithic breccias commonly overlie climactic ignimbrite/fallout deposits in small caldera-forming eruptions, and an alternative explanation is that this reflects the collapse of the shallow conduit after an eruption climax, whose walls had been highly fractured and had become unstable owing to progressive erosion.

3D geological modelling of the Renard 2 kimberlite pipe, Québec, Canada: from exploration to extraction

The Renard 2 kimberlite pipe is one of nine diamondiferous kimberlite pipes that form a cluster in the south-eastern portion of the Superior Province, Quebec, Canada and is presently being extracted at the Renard Mine. It is interpreted as a diatreme-zone kimberlite consisting of two Kimberley-type pyroclastic units and related country rock breccias, all cross-cut by coherent kimberlite dykes and irregular intrusives. Renard 2 has been the subject of numerous diamond drilling campaigns since its discovery in 2001. The first two geological models modelled kimberlite and country rock breccia units separately. A change in modelling philosophy in 2009, which incorporated the emplacement envelope and history, modelled the entire intrusive event and projected the pipe shape to depth allowing for more targeted deep drilling where kimberlite had not yet been discovered. This targeted 2009 drilling resulted in a > 400% increase in the volume of the Indicated Resource. Modelling only the kimberlite units resulted in a significant underestimation of the pipe shape. Current open pit and underground mapping of the pipe shape corresponds well to the final 2015 geological model and contact changes observed are within the expected level of confidence for an Indicated Resource. This study demonstrates that a sound understanding of the geological emplacement is key to developing a reliable 3D geological and resource model that can be used for targeted delineation drilling, feasibility studies and during the initial stages of mining.

Engineering geology model of the Crater Lake outlet, Mt. Ruapehu, New Zealand, to inform rim breakout hazard

Mt. Ruapehu, in the central North Island of New Zealand, hosts a hot acidic Crater Lake over the active volcanic vent with a surface elevation of c. 2530m.a.s.l. Volcanic activity and other montane processes have previously resulted in catastrophic releases of some or all of the c. 10Mm3 of water retained in the lake, creating serious hazards downstream. A major lahar in March 2007 exposed a 10m high face representative of the rock units impounding the lake, providing an opportunity to conduct both field and laboratory analysis to characterise the rock mass conditions at the outlet to assess the stability of the outlet area. This paper presents an engineering geology model of Crater Lake outlet. Our model shows three andesitic geological units at the outlet, each with different geological histories and physical and mechanical properties, which impact its stability.Geotechnical methods used to characterise the outlet include laboratory testing of the strength, stiffness, porosity and unit weight, and field-based rock mass characterisation using the geological strength index (GSI) and rock mass rating (RMR). Field observations, geomorphology mapping, historic and contemporary photographs, and laboratory results are combined to create cross sections that provide key information for establishing the engineering geology model. The units are recognised in what is informally termed the Crater Lake Formation: i) The upper surface layer is a c. 2m thick sub-horizontal dark grey lava unit (Armoured Lava Ledge) with sub-horizontal cooling joints spaced at 0.2m to 2.0m intervals. The intact rock has a porosity range of 15–27%, density range of 1723–2101kg/m3, GSI range of 45–75, and unconfined compressive strength (UCS) range of 19–48MPa. ii) Below this, and outcropping down the majority of the outlet waterfall is a poorly sorted breccia unit composed of block and matrix material (Lava Breccia). The blocks range from 0.1m to 0.8m in diameter with an average porosity of 21%, a density of 1956kg/m3 and strength of 85MPa. The matrix has soil-like properties with an estimated UCS of 1.5MPa. iii) At the base of the waterfall, the material sharply transitions into a light grey, slightly weathered unit (Lower Grey Member). This lower unit has an irregular surface expression with sub-vertical discontinuities. Porosity is 6%, density is 2569kg/m3, the GSI range is 65–75, and the UCS is 98MPa.The engineering geology model portrays the relationships between the units in three dimensions, highlights key structures and takes into consideration the material source, transportation and depositional processes. Historical outlet photographs suggest past eruptive and glacial activities are both significant factors controlling the deposition and erosion of material at the outlet. The Lower Grey Member appears to be a sound material for the outlet and water fall to be founded on. The upper aa Armoured Lava Ledge currently has moderate strength and GSI, and is resistive, providing protection for the underlying weaker block and matrix unit, however, continued incision by the outlet stream will eventually expose the weaker block and matrix material of the Lava Breccia. Once exposed, the Lava Breccia could rapidly erode or fail down to the Lower Grey Member and could potentially release 1Mm3 of hot, acidic Crater Lake water. We recommend that erosion rates for the upper Armoured Lava Ledge be established to aid in preparation for eventual rim breakout.

The Munali Ni sulfide deposit, southern Zambia: A multi-stage, mafic-ultramafic, magmatic sulfide-magnetite-apatite-carbonate megabreccia

The Munali Intrusive Complex (MIC) is a flattened tube-shaped, mafic-ultramafic intrusion located close to the southern Congo Craton margin in the Zambezi belt of southern Zambia. It is made up of a Central Gabbro Unit (CGU) core, surrounded by a Marginal Ultramafic-mafic Breccia Unit (MUBU), which contains magmatic Ni sulfide mineralisation. The MIC was emplaced into a sequence of metamorphosed Neoproterozoic rift sediments and is entirely hosted within a unit of marble. Munali has many of the characteristics of craton-margin, conduit-style, dyke-sill complex-hosted magmatic sulfide deposits. Three-dimensional modelling of the MUBU on the southern side of the MIC, where the Munali Nickel Mine is located, reveals a laterally discontinuous body located at the boundary between footwall CGU and hangingwall metasediments. Mapping of underground faces demonstrates the MUBU to have intruded after the CGU and be a highly complex, multi stage megabreccia made up of atypical ultramafic rocks (olivinites, olivine-magnetite rocks, and phoscorites), poikilitic gabbro and olivine basalt/dolerite dykes, brecciated on a millimetre to metre scale by magmatic sulfide. The breccia matrix is largely made up of a sulfide assemblage of pyrrhotite-pentlandite-chalcopyrite-pyrite with varying amounts of magnetite, apatite and carbonate. The sulfides become more massive towards the footwall contact. Late stage, high temperature sulfide-carbonate-magnetite veins cut the rest of the MUBU. The strong carbonate signature is likely due, in part, to contamination from the surrounding marbles, but may also be linked to a carbonatite melt related to the phoscorites. Ductile deformation and shear fabrics are displayed by talc-carbonate altered ultramafic clasts that may represent gas streaming textures by CO2-rich fluids. High precision U-Pb geochronology on zircons give ages of 862.39±0.84Ma for the poikilitic gabbro and 857.9±1.9Ma for the ultramafics, highlighting the multi-stage emplacement but placing both mafic and later ultramafic magma emplacement within the Neoproterozoic rifting of the Zambezi Ocean, most likely as sills or sheet-like bodies. Sulfide mineralisation is associated with brecciation of the ultramafics and so is constrained to a maximum age of 858Ma. The Ni- and Fe-rich nature of the sulfides reflect either early stage sulfide saturation by contamination, or the presence of a fractionated sulfide body with Cu-rich sulfide elsewhere in the system. Munali is an example of a complex conduit-style Ni sulfide deposit affected by multiple stages and sources of magmatism during rifting at a craton margin, subsequent deformation; and where mafic and carbonatitic melts have interacted along deep seated crustal fault systems to produce a mineralogically unusual deposit.

Petrogenesis and mineral characteristics of the oldest volcanogenic breccia unit from the Himalayan foreland basin, India

Jangalgali breccia unit (JBU), the oldest volcanogenic rock unit in the Himalayan foreland basin, has been studied from five different, however, stratigraphically equivalent localities of Jammu region in NW India. The field and thin section slides of JBU reveal that quartz and plagioclase are the two most common minerals and sanidine, magmatic zircon, rutile, hornblende and biotite are present as accessory phases. Petrographic signatures show that quartz and K-feldspar are set in a fine-grained cryptocrystalline glassy matrix and do not represent any preferred orientation. The euhedral hexagonal dipyramidal quartz phenocrysts, irregularly shaped inclusions having high volatile contents and no signatures of transport/reworking in phenocrysts prior to deposition are the prominent petrographic features of JBU. The mineral grains arrangement varies between tight-fitted fabric geometry to more open-chaotic packing. Overall, the field relationship, texture, mineralogy along with mineral chemistry, and presence of high gas and silica content in the host magma, support the volcanic origin of the JBU litho unit, which has wider implications towards understanding the timing of India-Asia collision as well as geodynamic evolution of the Himalayas.

Evidence for an impact-induced biosphere from the δ34S signature of sulphides in the Rochechouart impact structure, France

The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralisation. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ34S values vary over a wide range, from −35.8‰ to +0.4‰. The highest values, δ34S −3.7‰ to +0.4‰, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ34S −35.8‰ to −5.2‰, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ34S values of −10.7‰ to −1.2‰ are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120 °C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor.

Structure of a normal seismogenic fault zone in carbonates: The Vado di Corno Fault, Campo Imperatore, Central Apennines (Italy)

The Vado di Corno Fault Zone (VCFZ) is an active extensional fault cutting through carbonates in the Italian Central Apennines. The fault zone was exhumed from ∼2 km depth and accommodated a normal throw of ∼2 km since Early-Pleistocene. In the studied area, the master fault of the VCFZ dips N210/54° and juxtaposes Quaternary colluvial deposits in the hangingwall with cataclastic dolostones in the footwall. Detailed mapping of the fault zone rocks within the ∼300 m thick footwall-block evidenced the presence of five main structural units (Low Strain Damage Zone, High Strain Damage Zone, Breccia Unit, Cataclastic Unit 1 and Cataclastic Unit 2). The Breccia Unit results from the Pleistocene extensional reactivation of a pre-existing Pliocene thrust. The Cataclastic Unit 1 forms a ∼40 m thick band lining the master fault and recording in-situ shattering due to the propagation of multiple seismic ruptures. Seismic faulting is suggested also by the occurrence of mirror-like slip surfaces, highly localized sheared calcite-bearing veins and fluidized cataclasites. The VCFZ architecture compares well with seismological studies of the L'Aquila 2009 seismic sequence (mainshock MW 6.1), which imaged the reactivation of shallow-seated low-angle normal faults (Breccia Unit) cut by major high-angle normal faults (Cataclastic Units).

The Paroo Station Mine supergene lead deposits, Western Australia: Geological and geochemical constraints

The Paroo Station Mine lead project is situated near Wiluna in the central part of Western Australia. It is located in an outlier of the Paleoproterozoic Earaheedy Group overlying the southeast Yerrida Basin. The total Measured and Indicated Mineral Resource is currently (as at 31st of December 2014) estimated at 31.8Mt of ore at 4.4% Pb. The deposits are hosted by deeply weathered carbonate and clastic sediments of the Paleoproterozoic Yelma Formation. The lead-only mineralisation contains only traces of sulfides and is composed of few supergene minerals, predominantly cerussite and anglesite with minor pyromorphite and plumbogummite. The Paroo Station Mine demonstrates a high degree of supergene Pb mobility which has resulted in almost complete destruction of primary Pb distribution pattern and development of characteristic for the oxidation zones coupled depletion zone and subhorizontal enrichment blankets below.The deposits are interpreted to have formed from Mississippi Valley-type (MVT), stratabound relatively low-grade sulfide PbZn mineralisation located within dolomites of the Sweetwaters Well Member of the Yelma Formation. The primary mineralisation based on modelling the Pb distribution was controlled by a network of northeast, northwest and meridional oriented fractures and small faults. Lead isotope study yielded from 1.80 to1.88Ga ages of the mineralisation based on the Stacey and Kramers (1975) two-stage model; the mineralising event is related to the Capricorn Orogeny. This data is well in line with previous UPb dating of monazites interpreted to be related to the mineralisation (1.82Ga; Muhling et al., 2012). High μ (238U/204Pb) values (10.26–10.58) are indicative of the Archean basement contribution and/or more likely with sediments originally derived from that basement. The Paroo Station Mine mineralisation is thought to be similar in style and formed synchronously with the MVT prospects in the Teague area in the Earaheedy Basin approximately 120km to the northeast.The mineralisation at the Paroo Station Mine was significantly upgraded through a multi-stage supergene remobilisation and reprecipitation of lead in the course of long weathering history of the region. Lead was almost completely depleted from the upper zone and accumulated below as subhorizontal enrichment blankets at three levels (from top to bottom): (1) the base of the collapse quartz-clay breccia unit marking the dolomite weathering front; (2) the contact of siltstone and sandstone packages coincident with the feldspar/kaolinite weathering front and; (3) the mid- to lower parts of the sandstone unit close to the base of weathering. The lowest enrichment zone extends as a continuous blanket at a significant, up to 500m distance (at a 1% Pb cut-off) beyond the upper mineralisation projection. This enrichment is interpreted to be formed by lateral Pb transport in waters close to the groundwater table through the most permeable layers of weathered sandstones. This active lateral lead dispersion is of high exploration significance for the region providing a much larger exploration target and vector to the primary mineralisation.

Structural parameters of Cr-bearing spinels and pleonaste from the Cuillin Igneous Complex (Isle of Skye, Scotland): Implications for metamorphic and cooling history

AbstractThe Outer Layered Suite of the Cuillin Igneous Complex (Isle of Skye, NW Scotland) comprises a Peridotite Series and a younger Allivalite Series (the latter comprising troctolites, eucrites and gabbros). Close to the junction between the Peridotite and the Allivalite Series (but wholly contained within the latter), an ultramafic breccia unit containing abundant peridotite xenoliths crops out. In the Peridotite Series, reddish-brown Cr-bearing spinels are present as disseminated crystals in the peridotite and also as chromitite seams, while in the peridotite xenoliths of the breccia unit, green pleonaste occurs in both of these modes of textural occurrence. Optical absorption spectroscopy reveals that the colour difference between the two spinel phases is related mainly to variable Al, Cr and Fe contents, while crystal structural analysis shows that the cooling rate calculated utilizing the oxygen positional parameter is comparable for all samples. The intracrystalline closure temperature for the Cr-spinel in the Peridotite Series is different for the disseminated and seam textural occurrences of the spinels, while the temperatures yielded by pleonaste in the peridotite xenoliths are the same for both textural occurrences. Our dataset suggests that the pleonaste in the peridotite xenoliths has been heated and equilibrated under subsolidus conditions, probably during breccia formation. During this heating, homogenization of the closure temperatures of pleonaste spinels occurred.